G-protein-Coupled Receptors General Principles of Cell Signaling G-protein-Coupled Receptors Enzyme-Coupled Receptors Other Signaling Pathways GPCR G-protein-coupled receptors Figure 15-30 Molecular Biology of the Cell ( Garland Science 2008)
G-protein-coupled receptors (GPCR) The largest family of cell-surface receptors. More than 700 GPCRs in humans (1,000 concerned w/ the sense of smell alone in mice) Signals for GPCRs: hormones, local mediators, and neurotransmitters. A large variety of cellular process. An attractive target for drugs. About half of all known drugs work through GPCRs. Diversity of signals but, a similarity of GPCRs in structure. A single polypeptide chain Seven-pass transmembrane Rhodopsin (light activated photoreceptor protein in the vertebrate eye), the olfactory (smell) receptors in the vertebrate nose, mating receptors in yeast. Ancient, bacteriorhodopsin in bacteria possess similar proteins (do not act through G proteins) 1. Stimulation of GPCRs activates G-protein subunits
Stimulation of GPCRs activates G-protein subunits Signal binding to GPCRs > conformational change Activates a G protein G proteins 3 subunits: α, β, and γ α and γ tethered to membrane Unstimulated state: α subunit w/ GDP (idle) Ligand binding... Altered R activates a G protein (α subunit to decrease its affinity for GDP) Exchanging GDP to GTP. Breaking up the G protein subunits. α and βγ (two separate molecules) Stimulation of GPCRs activates G-protein subunits G-protein α subunit Limit the amount of time that α and βγ subunits remain switched on Limit availability to relay signals An intrinsic GTPase activity (hydrolyzes its bound GTP back to GDP) Returns the whole G protein to its original, inactive conformation GTP hydrolysis + Inactivation > within seconds after the G protein has been activated. Inactive G protein is now ready to be reactivated. The longer these target proteins have an α or βγ subunit bound to them, the stronger and more prolonged the relayed signal will be. Cholera ( ) Caused by a bacterium in the intestine Produces a cholera toxin Modifies α subunit G α no longer hydrolyzes its bound GTP Gs (because it stimulates adenylyl cyclase > [camp] ) Remains in the active state indefinitely Continuously transmitting a signal to its target proteins A prolonged and excessive outflow of Cl - and water into the gut (resulting in catastrophic diarrhea and dehydration) Often lead to death.
Stimulation of GPCRs activates G-protein subunits Pertussis ( 백일해 ) Whooping cough Infants now routinely vaccinated. A common respiratory infection The disease-causing bacterium colonizes the lung Produces a pertussis toxin Alters the α subunit Gi (because it inhibits adenylyl cyclase) Locking it into its inactive GDP-bound state Knocking out Gi > [camp ] Results in a prolonged signal (stimulates coughing) A general principle of cell signaling the mechanisms that shut a signal off are as important as the mechanisms that turn it on 2. Some G proteins directly regulate ion channels
Example for direct G-protein regulation of ion channels The target proteins for G protein subunits Membrane-bound enzymes Ion channels G-protein response 20 types of mammalian G protein Activated by a particular set of cell-surface receptors and a particular set of target proteins. Appropriate for that signal and that type of cell. The heartbeat is controlled by two sets of nerves heart up slows it down A slowdown signal in heartbeat. Acetylcholine (Ach) release Ach binds to a GPCR on the surface of the heart muscle cells GPCR activates G protein (Gi) βγ (active component) binds to the intracellular face of a K + channel Forces the ion channel into an open conformation Allows K + to flow out of the cells Inhibiting the cell s electrical excitability This signal is shut off - α subunit inactivates itself Example for direct G-protein regulation of ion channels 3. Some G proteins regulates the production of cyclic AMP Ach > G βγ > K + channel > Heartbeat
The most frequent target enzymes for G proteins Adenylyl cyclase (AC) produces cyclic AMP (camp) Phospholipase C (PLC) produces inositol triphosphate (IP 3 ) + diacylglycerol (DAG) *Second messengers The small intracellular signaling molecules generated in these cascades. They are produced in large numbers. In the resting nerve cells [camp] = 5 x 10-8 M [camp] = 10-6 M ACTH. [ ] adrenocorticotropic hormone
cyclic AMP pathway Frightened or excited, the adrenal gland releases the hormone adrenaline, which circulates in the bloodstream and binds to a class of G-proteinlinked receptors to help prepare the body for sudden action. camp Activated G protein α subunits (Gs) activates adenynyl cyclase Synthesized by adenylyl cylcase, degraded by camp phosphodiesterase Caffeine: phosphodiesterase in the nervous system > camp degradation > Keep [camp] high A dramatic and sudden increase in response to an extracellular signal camp phosphodiesterase is continuously active. It breaks camp down so quickly, rising or falling in a matter of seconds. Water-soluble (so it can carry its signal throughout the cell, traveling from the site on the membrane where it is synthesized to interact with proteins located in the cytosol, the nucleus, or other organelles) 4. The cyclic AMP-dependent protein kinase (PKA) mediates most of the effects of camp PKA (camp-dependent protein kinase) Normally held inactive in a complex with another protein. camp binding forces a conformational change that unleashes the active kinase. Activated PKA then catalyze the phosphorylation of particular serines or threonines on certain intracellular proteins (enzymes involved in glycogen metabolism or CREB) A rise in intracellular camp can also activate gene transcription. The binding of camp to the regulatory subunits induces a conformational change, causing these subunits to dissociate from the catalytic subunits, thereby activating the kinase activity of the catalytic subunits
1. Skeletal muscle cells (Glycogen breakdown; not gene transcription involved; rapid response) Adrenaline > GPCR > Gs > AC > [camp] > PKA > Phosphorylase kinase-p > Glycogen phosphorylase-p > glycogen breakdown or glycogen synthesis > more glucose available for muscular activity 2. Fat cells (not gene transcription involved; rapid response) Adrenaline > GPCR > Gs > [camp] > triglyceride breakdown > an immediately usage form of cell fuel 3. Endocrine cells or brain cells (gene transcription involved; slow response) Adrenaline > GPCR > Gs > [camp] > PKA > transcription regulator-p > activate gene transcription : hormone synthesis in endocrine cells or proteins involved in long term memory in the brain
5. Some G proteins activate an inositol phospholipid signalling pathway by activating phospholipase C-β Phospholipids and Glycolipids in the plasma membrane phosphatidylcholin (red), sphingomyelin (brown), phosphatidylserine (light green), phosphatidylinositol (dark green), and phosphatidylethanolamine (yellow); Glycolipids (blue hexagonal); cholesterol (gray).
Phospholipase C chops the sugar-phosphate head off the inositol phospholipid (generates two small signaling molecules). Inositol 1,4,5-triphosphate (IP3) + Diacylglycerol (DAG) Some GPCRs : G protein phospholipase C (PLC) inositol phospholipid pathway Once activated, PLC propagates the signal by cleaving a lipid molecule. Inositol phospholipid (a phospholipid with the sugar inositol attached to its head) Small quantities in the cytosolic half. Inositol phospholipid pathway. Operates in almost all eucaryotic cells Regulates a host of different effector proteins. Localized differently from all other glycolipids. IP 3 Water-soluble sugar phosphate that diffuses into the cytosol Relay the signal Binds to and opens special Ca 2+ channels in the ER membrane. Triggers the release of Ca 2+ from the ER Ca 2+ to rush out into the cytosol. DAG Remains embedded in the plasma membrane Relay the signal Helps recruit and activate protein kinase C (PKC) Recruited to the cytosolic face of the plasma membrane. Need Ca 2+ to become active. Phosphorylates its own set of intracellular target proteins, further propagating the signal.
6. Ca 2+ functions as a ubiquitous intracellular mediator The Ca 2+ wave provokes a change in the egg surface, preventing entry of other sperm, and initiates embryonic development An very important and widespread role as a messenger. [Ca 2+ ] in cytosol GPCRs Fertilized egg cells (When a sperm fertilizes an egg cell, Ca 2+ channels open and the resulting rise in Ca 2+ triggers the start of embryonic development) Skeletal muscle cells (a signal from a nerve triggers a rise in cytosolic Ca 2+ that initiates muscle contraction) Secretory cells (Ca 2+ triggers secretion)
Ca 2+ signal [Ca 2+ ] maintenance in the cytosol [Ca 2+ ] of an unstimulated cell : extremely low (10-7 M) compared with that in extracellular fluid and ER. Maintained by membrane-embedded pumps (actively pump Ca 2+ out of the cytosol-either into the ER or across the plasma membrane and out of the cell). A steep electrochemical gradient of Ca 2+ exists A signal transiently opens Ca 2+ channels (Ca 2+ rushes into cytosol). 7. Ca 2+ calmodulin-dependent protein kinases (CaM-Kinases) mediates many of the responses to Ca 2+ signals in animal cells
Ca 2+ signal The effects of Ca 2+ in the cytosol are largely indirect (mediated by Ca 2+ - responsive proteins) Ca 2+ -binding proteins (calmodulin, CaM) Present in the cytosol of all eucaryotic cells (mammals, plants, fungi, and protozoa) Binds to Ca 2+ (undergoes a conformational change that enables it to wrap around a wide range of target proteins, altering their activities). A target for CaM : Ca 2+ /calmodulin-dependent protein kinase (CaM-kinase) A neuron-specific CaM-kinase at synapses: learning and memory 8. Smell and vision depend on GPCRs that regulate cyclic nucleotide-gated ion channels Human can distinguish more than 10,000 district smells. Specialized olfactory receptor neurons in the linking of the nose. GPCR (olfactory receptors) is displayed on the surface of the cilia. Golf acts through camp. Odorant binding > Golf activation > Adenylyl cyclase activation > camp open camp-gated cation channels > influx of Na+ > depolarization of the olfactory receptor neuron > a nerve impulse that travels along its axon to the brain 1000 Golf in a mouse, 350 Golf in a human
Eye to bright light (photoreceptor): only 20 msec A rod photoreceptor cells in the retina cgmp-gated ion channels cgmp instead of camp Guanylyl cyclase and cyclic GMP phosphodiesterase controls cgmp concentraion. Rod receptor Light is sensed by rhodopsin (GPCR) Rhodopsin (GPCR) [dark] Gα > cgmp production > cation channel open > depolarisation > neurotransmitter release. [Light]Rhodopsin absorb photons > G > cgmp phosphodiesterase > cation channel closed > hyperpolarization > neurotransmitter reduced.
Intracellular mediators and enzymatic cascades amplify extracellular signals 9. GPCR desensitization depends on receptor phosphorylation Adaptability In bright sunlight (photons flood through each photoreceptor cell at a rate of billions per second) The signaling cascade adapts (stepping down the amplification more than 10,000-fold) Photoreceptor cells are not overwhelmed (still register + / - in the strong light) Negative feedback (an intense response generates an intracellular signal that inhibits the enzymes responsible for signal amplification) Frequently occurs in response to chemical signals Allow cells to remain sensitive to changes of signal intensity (allow a cell to respond to both messages, whispered and shouted).